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Repairing and restoring amplifiers.

Repairing and restoring amplifiers.

Often I read in all forums to replace all elco's.
Just like when you buy a car, than replace the interior.

Repairing an Amplifier:

If replacing the buffer elco's, one should replace them by a type of the same dimensions and voltages. Capacity than will be 2,5 up to 3 times as big. This will result in a higher and much stable DC voltage in loaded condition. 5% more output. More important, the bass will be deeper and straighter. Damping will increase.

What the most people forget is the most important thing:
The only elco's I always replace are couple elco's in the sound path.
These I replace by polyether or foil caps. (MKT, MKH, MKS or MKP)
The elco's in heat area have to be replaced by 105°C elco's.

Very important as well is, when repairing the power section. Replace transistors by modern types:
Higher frequency, more powerful, lower distortion, smaller internal capacity (oscillation) better slew rate. If ready adjust the idle current 20% higher. The amp will sound as twice his new price.

In Europe the line voltage has gone from 220 to 230V. In this case set the Amp to 240V if possible. If not, replace STK IC's by a higher voltage type. If there is no 240V possibility, put a 10W resistor 2,7Ω in series with the main line. This to provent damages in the future and will safe the power switch.

Re: Repairing and restoring amplifiers.

I would have to disagree with a lot of your post, if I am reading it correctly. Increasing filtering capacity in a power supply by 30-50% may be OK in solid state equipment; in tube power supplies it may stress the rectifiers too much. If you are talking about audio amplifier circuits, increasing values of capacitors by 2.5 to 3 times is NOT good; you are trying to re-engineer the circuit and there will be a change in frequency response and probably amplification; you may also have many unwanted consequences like distortion and poor linearity. '5% more output'? Maybe in low frequencies, with an additional distortion possibly factored in. In America, the term 'buffer' is used to describe isolation between one stage and the next; not sure what you mean when using this term.

If you want to go to all the trouble of replacing the power transistors, you might as well just buy another amp, if more power is your goal.

To 'save the power switch', you should just install a safety rated capacitor across it; this will reduce arcing.

Re: Repairing and restoring amplifiers.

"Often I read in all forums to replace all elco's."

What you read in all forums is wrong.

Re: Repairing and restoring amplifiers.

In programs like "Overhauling" and all alike, they replace drum brakes by disc brakes.
Carburetors by fuel injection, diagonal tires by radial tires. Infect they redesigning the car. They don't restore it for a museum, but for driving on the road. This is exactly what I am doing if the customer wants to pay for upgrading.
For 2 x 100W Amp, in the 70's we had 2 x 10.000µF, in the 80's 2 x 12.000µF.
Today 2 x 2x33.000µF is no problem at all rectifiers will do the job. In the 70's you would need a case of 21" x 21". Where most amps were 18" x 16". Not enough space.
105ºC caps didn't even exist just as the high quality transistors we got ten years later. In the 80's the manufacturers didn't use the old garbage but went for the new transistors, so why should I use this old junk when they are broke and spend lots of efforts to find them and pay an exceptional price. Instead of buy better, cheaper and easy to find transistors. Through my working history I was chief engeneer in a couple of servicenters manufacturers importer, as Akai, JVC, Fisher, Grundig and Marantz.I later had my own repair and servicenter.
Now retired I upgrade my income, by buying broke amps repair them and than sell them on ebay. I will never replace elco,s before measure them. The cheaper to repair the more I earn. If original transistor are in my possession NOS, so much the better. But there is one thing and that is called UPGRADING or Overhauling. To make it better than when it left the factory. If a costumer wants to pay, I will upgrade the amp to make it sound much better than when it left the plant. You can put me in a Museum, but not the amp.

Re: Repairing and restoring amplifiers.

I understand what you are saying about "Upgrading/Overhauling. I have run into a number of people who love their Fisher 500 and 800 Tube Receivers. There is a large group that continues to "Upgrade" these units with new and or modified circuits, some which for example will allow for adjusting the grid bias current individually for the 4 output tubes. They also replace all selenium rectifiers with silicon etc. All electrolytics are replaced, with lower value signal passing types replaced with high end poly or mylar.
But when I restore a vintage solid state unit, I want it to sound as good as the best one produced by the factory when new. I strive to recreate what the engineers of the 70's produced.
I replace usually all low value signal passing caps with high end type usually from nichicon, Panasonic, or others. I replace electrolyics with electrolytics testing all caps before installation to better match side to side. I replace most elecro caps in the regulator supplies because they usually overheat. I use 105 degrees when I can find them. I replace regulator transistor with higher rated ones as the originals have been running very hot for 40 years. I only replace filter caps when they read out of tolerance, or exhibit high ESR when measuring with my Peak Atlas cap/esr tester. I try not to mess with success. If a cap is still looking fine after 40 plus years, I don't want to put one in that may not last 10 years.
But I have done upgrades, and now do more and more, when it comes to visual items such as LED conversions for indicators as well as front panel illumination. The classy look you can now get using colored led's such as Green and Blue are really impressive when done on a Pioneer SX-980, or 1050.